1. Field of the Invention
This invention relates to the desulfurization and denitration of light oil by extraction.
2. Description of Prior Art
The term "light oil" means either an intermediate or a final product obtained from the process of petroleum refining. Light oil as an intermediate product usually contains about 1% by weight of sulfur compounds. The sulfur compounds not only exert an adverse effect on the quality of petroleum products, but also form as a result of combustion sulfur oxides which cause environmental pollution. Light oil is, therefore, desulfurized to make a wide range of products including a cleanser, a fuel for a diesel engine, or burner, absorption oil, oil gas, and thermally or catalytically cracked gasoline.
The removal of sulfur compounds from light oil has hither-to been effected almost exclusively by hydrodesulfurization. The hydrodesulfurization of light oil is effected at high temperature in the range of about 280.degree. to 340.degree. C. and a high pressure in the range of about 20 to 50 bars in the presence of a catalyst, e.g. a cobalt-molybdenum catalyst on a support of alumina, to remove sulfur compounds by converting them to hydrogen sulfide and hydrocarbons.
The conventional process of hydrodesulfurization as hereinabove described enables a reduction in the sulfur content of light oil to a level of 0.07 to 0.08% by weight, and can, therefore, satisfy the presently existing regulation which specifies an upper limit of 0.5% by weight for the sulfur content of light oil. It is, however, expected that a new upper limit of 0.05% by weight will be set in the near future for the purpose of e.g. environmental protection, and if such is the case, the conventional process will become useless. From a technical standpoint, it is possible to obtain a hydro-desulfurized product of light oil having a sulfur content not exceeding 0.05% by weight, but for that purpose, it is necessary to employ by far higher temperature and pressure than have hitherto been employed, and therefore to use new equipment and larger amounts of energy and hydrogen. Moreover, the hydrodesulfurized product has a black color which has to be removed before it can be a commercially desirable product. This color becomes more remarkable with a reduction in the sulfur content of the product. It also has an offensive smell. These problems make it undesirable as a commercially acceptable product.
In addition, light oil contains nitrogen compounds in concentration of from about a hundred to several hundreds ppm. As the nitrogen compounds form as a result of combustion NO.sub.M which causes environmental pollution, it is desirable to remove said nitrogen compounds from light oil as much as possible. But the efficient denitration of light oil has not been reported.